133 research outputs found
Supported bilayers: combined specular and diffuse x-ray scattering
A new method is proposed for the analysis of specular and off-specular
reflectivity from supported lipid bilayers. Both thermal fluctuations and the
"static" roughness induced by the substrate are carefully taken into account.
Examples from supported bilayers and more complex systems comprising a bilayer
adsorbed or grafted on the substrate and another "floating" bilayer are given.
The combined analysis of specular and off-specular reflectivity allows the
precise determination of the structure of adsorbed and floating bilayers, their
tension, bending rigidity and interaction potentials. We show that this new
method gives a unique opportunity to investigate phenomena like protusion modes
of adsorbed bilayers and opens the way to the investigation of more complex
systems including different kinds of lipids, cholesterol or peptides
Solid like friction of a polymer chain
We propose a simple friction model for isolated polymer chains on a solid
substrate. The chains are pulled at constant velocity by one end, the other end
can be trapped on the solid substrate on localised sites. We focus on the
energy dissipation due to the traps. This simple model leads to non trivial
friction laws, depending on the velocity and the distance between traps. Some
refinements of the model such as the effect of thermal fluctuation are also
reported.Comment: 16 pages, 4 eps figures, accepted for publuication in Eur. Phys. J. E
New version of 20/07/2000 minor modifications to figure
Comment on `About the magnetic field of a finite wire'
A flaw is pointed out in the justification given by Charitat and Graner [2003
Eur. J. Phys. vol. 24, 267] for the use of the Biot--Savart law in the
calculation of the magnetic field due to a straight current-carrying wire of
finite length.Comment: REVTeX, 3 pages. A slightly expanded version that has been accepted
for publication by Eur. J. Phy
Scattering from Solutions of Star Polymers
We calculate the scattering intensity of dilute and semi-dilute solutions of
star polymers. The star conformation is described by a model introduced by
Daoud and Cotton. In this model, a single star is regarded as a spherical
region of a semi-dilute polymer solution with a local, position dependent
screening length. For high enough concentrations, the outer sections of the
arms overlap and build a semi-dilute solution (a sea of blobs) where the inner
parts of the actual stars are embedded. The scattering function is evaluated
following a method introduced by Auvray and de Gennes. In the dilute regime
there are three regions in the scattering function: the Guinier region (low
wave vectors, q R << 1) from where the radius of the star can be extracted; the
intermediate region (1 << q R << f^(2/5)) that carries the signature of the
form factor of a star with f arms: I(q) ~ q^(-10/3); and a high wavevector zone
(q R >> f^(2/5)) where the local swollen structure of the polymers gives rise
to the usual q^(-5/3) decay. In the semi-dilute regime the different stars
interact strongly, and the scattered intensity acquires two new features: a
liquid peak that develops at a reciprocal position corresponding to the
star-star distances; and a new large wavevector contribution of the form
q^(-5/3) originating from the sea of blobs.Comment: REVTeX, 12 pages, 4 eps figure
Effect of an electric field on a floating lipid bilayer: a neutron reflectivity study
We present here a neutron reflectivity study of the influence of an
alternative electric field on a supported phospholipid double bilayer. We
report for the first time a reproducible increase of the fluctuation amplitude
leading to the complete unbinding of the floating bilayer. Results are in good
agreement with a semi-quantitative interpretation in terms of negative
electrostatic surface tension.Comment: 12 pages, 7 figures, 1 table accepted for publication in European
Physical Journal E Replaced with with correct bibliograph
Controlling interactions in supported bilayers from weak electrostatic repulsion to high osmotic pressure
Understanding interactions between membranes requires measurements on
well-controlled systems close to natural conditions, in which fluctuations play
an important role. We have determined, by grazing incidence X-ray scattering,
the interaction potential between two lipid bilayers, one adsorbed on a solid
surface and the other floating close by. We find that interactions in this
highly hydrated model system are two orders of magnitude softer than in
previously reported work on multilayer stacks. This is attributed to the weak
electrostatic repulsion due to the small fraction of ionized lipids in
supported bilayers with a lower number of defects. Our data are consistent with
the Poisson-Boltzmann theory, in the regime where repulsion is dominated by the
entropy of counter ions. We also have unique access to very weak entropic
repulsion potentials, which allowed us to discriminate between the various
models proposed in the literature. We further demonstrate that the interaction
potential between supported bilayers can be tuned at will by applying osmotic
pressure, providing a way to manipulate these model membranes, thus
considerably enlarging the range of biological or physical problems that can be
addressed.Comment: 14 pages, 8 figure
Swelling of phospholipid floating bilayers: the effect of chain length
The equilibrium distance between two lipid bilayers stable in bulk water and
in proximity of a substrate was investigated. Samples consisted of a
homogeneous lipid bilayer, floating near an identical bilayer deposited on the
hydrophilic surface of a silicon single crystal. Lipids were saturated di-acyl
phosphocholines, with the number of carbon atoms per chain, n, varying from 16
to 20. The average and r.m.s. positions of the floating bilayer were determined
by means of neutron specular reflectivity. Samples were prepared at room
temperature (i.e. with the lipids in the gel phase) and measurements performed
at various temperatures so that the whole region of transition from gel to
fluid phase was explored. Data have been interpreted in terms of competition
between the interbilayer potential and membrane fluctuations and used to
estimate the bending rigidity of the bilayer
Self healing slip pulses along a gel/glass interface
We present an experimental evidence of self-healing shear cracks at a
gel/glass interface. This system exhibits two dynamical regimes depending on
the driving velocity : steady sliding at high velocity (> Vc = 100-125 \mu
m/s), caracterized by a shear-thinning rheology, and periodic stick-slip
dynamics at low velocity. In this last regime, slip occurs by propagation of
pulses that restick via a ``healing instability'' occuring when the local
sliding velocity reaches the macroscopic transition velocity Vc. At driving
velocities close below Vc, the system exhibits complex spatio-temporal
behavior.Comment: 4 pages, 6 figure
From supported membranes to tethered vesicles: lipid bilayers destabilisation at the main transition
We report results concerning the destabilisation of supported phospholipid
bilayers in a well-defined geometry. When heating up supported phospholipid
membranes deposited on highly hydrophilic glass slides from room temperature
(i.e. with lipids in the gel phase), unbinding was observed around the main gel
to fluid transition temperature of the lipids. It lead to the formation of
relatively monodisperse vesicles, of which most remained tethered to the
supported bilayer. We interpret these observations in terms of a sharp decrease
of the bending rigidity modulus in the transition region, combined
with a weak initial adhesion energy. On the basis of scaling arguments, we show
that our experimental findings are consistent with this hypothesis.Comment: 11 pages, 3 figure
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